Chapter 25 Metabolic Pathways for Lipids and Amino Acids 25.1 Digestion of Triacylglycerols 25.2 Oxidation of Fatty Acids 25.3 ATP and Fatty Acid Oxidation

Digestion of Triacylglycerols In the digestion of fats (triacylglycerols): Bile salts break fat globules into micelles in the small intestine. Pancreatic lipases hydrolyze ester bonds to form monoacylglycerols and fatty acids, which recombine in the intestinal lining. Lipoproteins form and transport triacylglycerols to the cells of the heart, muscle, and adipose tissues.

Digestion of Triacylglycerols

Fat Mobilization Fat mobilization: Breaks down triacylglycerols in adipose tissue to fatty acids and glycerol. Fatty acids are hydrolyzed initially from C1 or C3 of the fat. Triacylglycerols + 3H2O Glycerol + 3Fatty acids

Fatty Acid Activation Fatty acid activation: Allows the fatty acids in the cytosol to enter the mitochondria for oxidation. Combines a fatty acid with CoA to yield fatty acyl CoA that combines with carnitine.

Fatty Acid Activation Fatty acyl-carnitine transports the fatty acid into the matrix. The fatty acid acyl group recombines with CoA for oxidation.

Fatty Acid Activation Fatty acid activation is complex, but it regulates the degradation and synthesis of fatty acids.

Beta-Oxidation of Fatty Acids In reaction 1, oxidation: Removes H atoms from the  and  carbons. Forms a trans C=C bond. Reduces FAD to FADH2.  

Beta-Oxidation of Fatty Acids In reaction 2, hydration: Adds water across the trans C=C bond. Forms a hydroxyl group (—OH) on the  carbon.  

Beta ()-Oxidation of Fatty Acids In reaction 3, a second oxidation: Oxidizes the hydroxyl group. Forms a keto group on the  carbon.  

Beta ()-Oxidation of Fatty Acids In Reaction 4, acetyl CoA is cleaved: By splitting the bond between the  and  carbons. To form a shortened fatty acyl CoA that repeats steps 1 - 4 of -oxidation.

Beta ()-Oxidation of Myristic (C14) Acid

Beta ()-Oxidation of Myristic (C14) Acid (continued) 7 Acetyl CoA 6 cycles

Cycles of -Oxidation The length of a fatty acid: Determines the number of oxidations and The total number of acetyl CoA groups. Carbons in Acetyl CoA -Oxidation Cycles Fatty Acid (C/2) (C/2 –1) 12 6 5 14 7 6 16 8 7 18 9 8

-Oxidation and ATP Activation of a fatty acid requires: 2 ATP One cycle of oxidation of a fatty acid produces: 1 NADH 3 ATP 1 FADH2 2 ATP Acetyl CoA entering the citric acid cycle produces: 1 Acetyl CoA 12 ATP

ATP for Lauric Acid C12 ATP production for lauric acid (12 carbons): Activation of lauric acid -2 ATP 6 Acetyl CoA 6 acetyl CoA x 12 ATP/acetyl CoA 72 ATP 5 Oxidation cycles 5 NADH x 3ATP/NADH 15 ATP 5 FADH2 x 2ATP/FADH2 10 ATP Total 95 ATP